docs: def _get_slope(self):
          """Return a float of the increase in y divided by the increase in x,
          both from left to right."""
          orig = self.origin
          dest = self.destination
          ox = orig.x
          oy = orig.y
          dx = dest.x
          dy = dest.y
          if oy == dy:
              return 0
          elif ox == dx:
              return None
          else:
              rise = dy - oy
              run = dx - ox
              return rise / run